In work reported previously, we had found low levels of radioactivity associated with the immunoprecipitates (IP) obtained by treatment of extracts of H3-uridine labelled SV40 transformed mouse embryo cells with hamster anti-T antigen serum (T sample). Such radioactivity was not observed in samples obtained following treatment of such extracts with buffer (control, N samples). This work was repeated. Although more tissue culture flasks of cells were used in each experiment, essentially the same results were obtained, even after extensive washing of the IP under a variety of conditions. These results appeared to confirm those obtained earlier. The levels of radioactivity observed were low, however, and presented the necessity of distinguishing between contamination of the IP by unincorporated H3-uridine (artifact), or low levels of RNA synthesis (real incorporation).